Although Microsoft claims that the new Entity Framework is really not an OR/M product, if you look closely at the examples and the whitepapers, you'll agree with me that they are very similar. Nhibernate is an awesome product, and I've been using it for a long time now, but it currently has two major drawbacks. First, there are no serious GUI tools assisting in the creation of mapping files and/or business classes. Second, various rumours indicate that further development (other than bugfixes) is uncertain at this time. Well, if you tried the new Entity Framework tools for Visual Studio 2008, you must love the integrated environment. And obviously, LINQ is much better than NHibernate's expression object model and HQL language. Nevertheless, until we have support for the major important database vendors, the EF is no real competiter yet. Luckely, there is good news on the horizon.
PS. If you have problems trying to get the CTP tools to work, see this post. It might help.